Apparatus for reducing noise in microphone circuits



Sept. 28, 1948. a J. F. ANDREWS 6 2,449,971

APPARATUS FOR REDUCING NOISE IN MICROPHONE CIRCUITS Filed Feb. 26, 1944INVENTOR JOSEPH FRANK ANDRE ws.

Patented Sept. 28, 1948 uN TEDfSTAT I Appucamnfr mm 2s, 1944,; SerialNo. 523.993.. 1' cam ici. 175 1) (Grantee under-t em of Mai-chd, ate,it";

-- amended April 30, 1928; 0,6. 751) This invention relatesto'microphone or speech input circuits of communication systems and hasparticular reference to radio communication systems for aircraft.

In currently designed aircraft communication systems of the classdescribed and particularly where oxygen mask equipment is utilized,.muchdistortion of speech in the receivers has-beenfound to exist. Thisdistortion is caused pri: marily by too high an amplification of thlower frequency bands of speech frequency components. The components ofspeech frequency in the lower bands i. e. those below about 350 cycles,approach the natural resonant frequency of the various voice cavitieswhich include the voice passages of the throat and the cavities in theoxygen mask. Because of this, sympathetic vibration of such cavities attheir natural resonant frequency is set up and fed into the speechamplifier at too high an amplitude level and thereby produce distortionin the speech when it is received. Furthermore, certain other lowfrequency noises which are inherent in the operation of the aircraftfind their way into the speech amplifier to produce additionaldistortion. Also present are high frequency noises i. e. those aboveabout 3,000 cycles, and these likewise are also stepped up to too high alevel in the speech amplification and interfere with intelligibility ofthe speech as it is received.

Since the naturalness of one's voice is determined primarily by thosecomponents of speech frequency which lie between about 350 and 3,000cycles, and which may be referred to as the essential voice frequencyband, it is evident that much 7 of the aforesaid distortion can beeliminated by providing a system in which frequencies below and abovethe aforesaid limits are substantially attenuated.

The primary object of this invention is therefore to provide a novelmicrophone input circuit for communication systems in which substantialattenuation is obtained with respect to frequencies which lie below andabove predetermined limits.

A more specific object of the invention is to provide an input circuitof the class described which comprises a series connected capacitor anda parallel connected varistor having a capacitor in series therewith,the function of which is to substantially attenuate frequencies whichlie both above and below predetermined limits,

These and other objects will become more apparent from the detaileddescription to follow and from the accompanying drawing, th singlefigureof which is a schematic circuit diagram ,of a preferredembodiment-of the-invention. I 1

Referring now to the drawing, the speech in put circuit includesamicrophone III which is connected in circuitwith a battery. H andprimarylwindingllof step up transformer l3...

The secondary .wind ing 14 of transformer I3 is connected at one sidethereof. tothegrid, l5 of,amplifl er-.-tube l6. The othersideof the;sec,-

. y winding is connected to the cathode, |1 oftube it... U

The anode ll of tube i6 is connected in cir cuit with a source ofpotential I9 and the primary winding 20 of step-up transformer 2 i. Thesecondary winding 22 of transformer 2| is connected to otheramplification stages if desired and thence fed into the telephonereceivers. Such parts of the circuit are conventional and hence have notbeen illustrated as the invention described and claimed herein isdirected to novel improvements in that portion of the speech inputcircuit which is shown in the drawing.

A capacitor 23 is connected in series in the speech input circuit and isof such capacity as to present a very high impedance to frequenciesbelow about 350 cycles but which decreases toward the normal impedancewithin the essential voice frequency band which lies above thisfrequency. Thus the gain of tube I6 is reduced materially with respectto speech frequency components below 350 cycles.

Connected in parallel with the secondar M of transformer I3 is a secondcapacitor 24 and a varistor 25 in series therewith. Varistor 25 has anegative resistance characteristic. One suitable type which may be usedis made from silicon carbide and is described in the October 1940 issueof the Bell Laboratory Record. The resistance characteristic of thevaristor is such that its value decreases exponentially as the potentialimpressed across it is increased. Thus a very small change in thepotential impressed across the varistor will result in a considerablechange in current through it. The function of varistor 25 and capacitor24 in series therewith is to present a high shunt impedance toallcomponents of speech frequencies below about 3,000 cycles. However,with respect to frequencies which lie above 3,000 cycles-and which arenormall at too high a level of signal potential, the impedance presentedby capacitor 24 and varistor 25 drops appreciably and hence increasesthe shunting effect of capacitor 24 to reduce the gain of tube IS.

A parallel connected resistor 26 may also be 3 utilized and functions tofurnish grid bias to tube I5.

While the proper values of capacity and resistance of the various partsshown in the drawing will vary somewhat with the type of microphone,transformers"aud t-amplifier tubesguse'd tin ,thei'-: system, Ihavefo'und that'very good results =may be obtained when the capacity ofcapacitor 23 apparatus used with an oxygen mask for attenuating soundsimpinging on a, microphone therein, which sounds have a, high magnitudeat frequencies above and below the essential speech frequency band, thecombination of a circuit for userwithztsuoh apparatus including 12a:first capacitdn-ofifsuchtsizeiasxfto offer :a:?high"- impedance tofrequencies of the order of 350 cycles and below,

is .025 mf., that of capacitor 24 .008 mf., that? means having anegative resistance characterisresistance of varistor 25variable"betweennlfinfln iltiohsaidameans.b i in Series W a Second and10,000 ohms, and the resistance oniresistori:' ana'citbri'i saidm ansand said se nd capa itor 26 2 megohms. being inishunt across saidcircuit for efiecting a In conclusion, it should be evidentstnatlowrimncdancec hcncc ase shuntin novel circuit arrangementwkrichgha-sl-been.ide-i effect to signals of high, magnitude atfrequencies scribed will function to attenuate" substantialiy*'15rofthe-order'of- 3000*cyclcs n above, whereby those voltages impressedacross the grid'il5a-iof the eoutput of the circuit has an essentiallyflat frequency response within the speech frequency quencies which liebelow aboutr'350 cycleszand'ifl and-:2 above about 3,000 cycles, withthe result "that speech input to the amplifier tube Hi from the ,20microphone r l Wis? amplifid iand transmitted *td f the various receiversin the 'systemrwith very-little distort/lone Th inventiozi descflbdhereirr may bein'a'nn fa cturedrand used by jor for the Goyernmenuon 2JOSEPH FRANK ANDREWS.

' REFERENGESvCITEDzE file 'ofithi's patent r V a UNITEDSTATESUPATENTSH'rrietrcnswmg:references:cr of record in the the unitcd 'statescrmmencagrefg vemmentcr N'lirhbcr Name Date pu-rpq'seg -witfibut, 'thepa ymen'tf ofz fany royafltigs f J 1 1 D965 5;E 1933 "I m a r w1,90%,762 Bierworth-; Apr. 16,-1935- Havingithus-" fully describedmyinventionril 2,930,937 n Feb; 1936 wy n -l I 30"? 2,04 -3;-16-1" FosterJune '2, 1936* In animproved distortionless communication

